In the cement industries it is known to use CaSO4 from gypsum to control the setting properties of cement paste. Synthetic gypsum from flue gas desulphurization (FGD) has become a prime source of gypsum as central power producing stations that burn coal have been converted to the use of the desulphurization process. As a result synthetic gypsum is a major sulphate donor for control of the setting properties of cement. The synthetic gypsum available from FGD processes usually has a sludge-like consistency that is very sticky and quite difficult to handle with accurate predictability. One problem that the difficult handling causes is the control of the quantity of pure synthetic gypsum in a cement mix so that fluctuations in the cement clinker/sulphate ratio do not occur and adversely affect the quality of the cement. Accordingly, one general object of the present invention is to transform the synthetic gypsum into a form which makes its handling easy and its metering reliable.
In a patent entitled “Process for the Manufacture of Lump Calcium Sulfate” which issued on Nov. 6, 1979 to Huller, et al. as U.S. Pat. No. 4,173,610, a process for the manufacture of calcium sulfate pellets from finely divided, natural or synthetic calcium sulfate is described where the free water content of the calcium sulfate is about 0.5 to about 4 percent and then the finely divided calcium sulfate is pressed in a roller press between 0° and 60° C. However, it is an object of the present invention to provide synthetic calcium sulphate by a simplified process that does not require the step of making finely divided pellets or the drying of calcium sulfate by external heating or drying means.
In another patent entitled “Process for Producing Gypsum Flake from Flue Gas Desulfurization” which issued on Nov. 8, 1994 to Roth, et al. and was given U.S. Pat. No. 5,362,471, flakes or chips of gypsum are formed from powdered gypsum whose moisture content is adjusted and then the gypsum is compacted and rolled into a sheet. Accordingly, it is another object of the present invention to avoid the use of powdered gypsum and the addition of moisture to produce gypsum flake.
In U.S. Pat. No. 6,471,767 B1 which issued on Oct. 29, 2002 to Konczak, wet synthetic gypsum waste is dewatered by passing it through an extruder and draining the water from it. The dewatering is aided by applying a vacuum to the extruder. This process produces synthetic gypsum rock and requires complicated extrusion and vacuuming processes that have high energy consumption.
Accordingly, it is another object of the present invention to provide a process for making synthetic gypsum in solid form which does not require the use of an extruder or vacuum and has relatively low energy consumption.
The foregoing and other objects are achieved by the present invention as will be better understood by reference to the following summary of the invention, drawings, and detailed description.